Vehicle transmissions are designed to transmit torque from an engine to a set of drive wheels in order to propel the vehicle through a range of output speeds. The engine output shaft may be selectively connected to a transmission input shaft when engine propulsion is required. In a manual transmission, a clutch pedal may be depressed to allow a driver to shift gears and/or to place the transmission into a neutral state. A hydrodynamic torque converter automatically provides the engine/transmission connection in an automatic transmission.
A torque converter includes an impeller/pump, a turbine, and a stator. The torque converter is filled with oil. The pump, which may be bolted to a rotating engine flywheel to thereby continuously rotate at engine speed, discharges the oil into the turbine through the stator. The fluid-driven turbine is connected to the transmission input shaft, and therefore rotation of the turbine ultimately causes a rotation of the coupled transmission input shaft. A variable level of slip occurs between the pump and turbine sides of the torque converter below a threshold lockup speed.
Use of a torque converter and the variable slip it provides thus enables a fluid coupling effect to occur between the engine and the transmission. This allows the vehicle to slow to a stop without stalling, while also allowing required torque multiplication to occur at low vehicle output speeds. In some vehicle transmissions the engine can be automatically shut off while the vehicle is stationary, e.g., while parked at a traffic light with the brake pedal applied, to minimize idle fuel consumption. After restart and launch, the torque converter can provide such torque multiplication to enhance the drive response at low speeds.